A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a vehicle carries a navigation function. Such a navigation system detects the position of the user or user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium in the navigation system. The navigation system displays a map image on a monitor screen while superimposing a mark representing the current location of the user on the map image. When a destination is set, the navigation system starts a route guidance function for setting a guided route from the start point to the destination and guides the user to the destination.
FIGS. 1A-1H show an example of overall procedure and screen display involved in the navigation system. FIG. 1A shows an example of locator map screen of the navigation system typically used when the destination is not specified. The navigation system displays a street on which the vehicle (current vehicle position VP) is running on a map image and a name of the street. Other information such as a north pointer NP, a map scale and a current time may also be illustrated on the display screen.
FIGS. 1B-1F show an example of process for specifying a destination in the navigation system. When selecting a “Destination” menu on a main menu screen of FIG. 1B, the navigation system displays an “Find Destination By” screen as shown in FIG. 1C listing various method for selecting the destination. When selecting, for example, the “Address” method, the navigation system displays a keyboard screen in FIG. D. Suppose the user inputs a particular address, the navigation system displays a “Confirm Destination” screen such as shown in FIG. 1E. If the name and address on the screen shows the correct destination, the user enters an “OK to Proceed” key to proceed to the next procedure.
In FIG. 1F, the navigation system calculates and determines a route to the destination, i.e., the specified address. The navigation system determines a route to the destination based on, for example, the quickest way to reach the destination, the route using many free ways as possible or the route without using toll road, and the like. Once the route to the destination is determined, the navigation system starts the route guidance as shown in FIG. 1G for guiding the user to the destination. Typically, the navigation system shows the intersection to show the next turn and a direction of the turn. Such route guidance by the navigation system is also accompanied by voice instructions.
The navigation system measures a distance to the current destination to detect whether the destination is within a predetermined range such as 100 meters. When the user approaches the destination within such a predetermined distance, the navigation system indicates that the destination “Dest” is ahead as shown in FIG. 1H. The navigation system detects the actual arrival based on various factors such as the engine stop.
The user can reach the destination by following the calculated route obtained by the navigation system. A problem arises when the user fails to follow the calculated route. FIG. 2A is a schematic diagram showing a calculated route to be followed to the destination. The alphabet letters indicate various road segments involved in the surrounding area of the calculated route. In this example, the user is supposed to drive along the road segment “h” and then make a left turn to the road segment “a” to stay on the calculated route.
FIG. 2B is a schematic diagram showing a condition when the user fails to follow the calculated route by making a right turn to enter the road segment “b”. The navigation system generally has an off-route detection mechanism that detects when the vehicle has deviated from the calculated route to the destination. If the off-route detection mechanism has determined that the vehicle has deviated from the calculated route, i.e., off-route, the navigation system starts a recalculation process to create a new route to the destination.
However, the recalculation may not always result in a safe and accurate route to the destination. Referring to FIG. 2C, the schematic diagram shows a condition where the new route is calculated but a maneuver by the user for taking the new route is problematic. For example, it is assumed that the navigation system has calculated a route which goes from the road segment “b” and then to the road segment “i”. However, if the navigation system finishes the route calculation when the vehicle is located at the position indicated by VP1 and guides the user for the new route, the user may not have a sufficient time to maneuver the vehicle to make a left turn to take the road segment “i” because the intersection is too close to the vehicle position. Even if the user could manage to enter into the road segment “i”, the maneuver may force the user to make a hasty and unsafe move to make the left turn.
Moreover, there is also a problem that the navigation system may continuously perform recalculation for a new route if the vehicles has passed a new maneuver point before the navigation system recalculates the new route. For instance, referring to FIG. 2C, if the navigation system recalculates a new route that prompts the user to make a left turn to the segment “i” but the vehicle has already passed the turning point and is now located at VP2 on the segment “c”, the recalculated route becomes useless and the navigation system starts to recalculate again to find a new route. In a worst case scenario, the navigation system may keep repeating recalculation for a new route, thus, no route guidance is provided to the user for a long period of time.
If the navigation system is to take a maneuver point that is too distanced from the present vehicle location, the above-noted problem of repeated recalculation may be prevented. However, the chance of traveling an inefficient route is increased. Moreover, the farther the maneuver point is set, the more difficult it becomes to predict whether the vehicle will actually come to the maneuver point. Thus, there is a need of a new method which can efficiently recalculate a new route when the vehicle has deviated from the calculated route.